Lean body mass and skeletal muscle mass decline with aging. Nonetheless, on the whole, the parameters of body protein metabolism are much the same in young and elder adults between the ages of 60 and 75 years when the data are expressed per unit lean mass. Healthy elderly adults are capable of maintaining nitrogen balance at protein intakes consistent with the young adult requirement level and, in general, healthy older subjects can reestablish nitrogen balance in a fashion similar to young adults when dietary protein intake is reduced. Protein turnover can be characterized with stable isotopically labeled amino acids and mass spectrometry. When corrected for lean mass, whole body protein turnover in healthy elderly adults is no different from that measured in younger adults. Nevertheless, the proportional contribution of non-muscle tissues to whole body protein turnover is increased since the fractional muscle protein synthesis rate declines with age. This reduction in muscle protein synthesis, however, is not simply a consequence of aging per se because the rate returns to a young adult value when aging muscles work against resistance. Further, exercise training improves muscle mass and strength in older adults. The age-dependent diminution in growth hormone secretion is not likely the cause of reduced muscle mass in the elderly since the beneficial effects of resistance exercise training along on muscle function are not enhanced by supplemental growth hormone treatment.